Derive a unique per-job seed for minicalosim shard generation

Concurrent job launches in create_root_files.py can start within the
same wall-clock second, and minicalosim's default seed falls back to
time(NULL) in that case — so two "independent" shards could silently
get identical RNG state and produce byte-identical physics. Requires
the companion MINICALOSIM_SEED env-var support in the minicalosim repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_create_root_files.py

@@ -9,6 +9,7 @@ from scripts import create_root_files
 parse_detector_spec = create_root_files.parse_detector_spec
 next_shard_index = create_root_files.next_shard_index
 plan_jobs = create_root_files.plan_jobs
+job_seed = create_root_files.job_seed
 run_job = create_root_files.run_job
 run_all = create_root_files.run_all
 SimJob = create_root_files.SimJob
@@ -35,7 +36,13 @@ start = time.time()
 time.sleep({sleep})
 end = time.time()
 payload = json.dumps(
-    {{"argv": sys.argv[1:], "cwd": os.getcwd(), "start": start, "end": end}}
+    {{
+        "argv": sys.argv[1:],
+        "cwd": os.getcwd(),
+        "start": start,
+        "end": end,
+        "seed": os.environ.get("MINICALOSIM_SEED"),
+    }}
 )
 for i in range({output_count}):
     with open(f"out_{{i}}.root", "w") as f:
@@ -130,6 +137,42 @@ def test_plan_jobs_multiple_detectors_each_start_independently(tmp_path):
     assert by_detector["sampling_fe_scint"] == [0, 1]
 
 
+def test_job_seed_deterministic():
+    job = SimJob(detector="pbwo4", config=None, shard_index=3)
+    assert job_seed("steps", "gen1", job) == job_seed("steps", "gen1", job)
+
+
+def test_job_seed_varies_by_shard_index():
+    a = SimJob(detector="pbwo4", config=None, shard_index=0)
+    b = SimJob(detector="pbwo4", config=None, shard_index=1)
+    assert job_seed("steps", "gen1", a) != job_seed("steps", "gen1", b)
+
+
+def test_job_seed_varies_by_detector():
+    a = SimJob(detector="pbwo4", config=None, shard_index=0)
+    b = SimJob(detector="sampling_pb_scint", config="pb_scint", shard_index=0)
+    assert job_seed("steps", "gen1", a) != job_seed("steps", "gen1", b)
+
+
+def test_job_seed_varies_by_gen():
+    job = SimJob(detector="pbwo4", config=None, shard_index=0)
+    assert job_seed("steps", "gen1", job) != job_seed("steps", "gen2", job)
+
+
+def test_run_job_passes_deterministic_seed_env_var(tmp_path):
+    fake = _write_fake_executable(tmp_path / "fake_exe.py")
+    (tmp_path / "raw" / "steps" / "gen1").mkdir(parents=True)
+    tmp_root = tmp_path / ".sim-tmp"
+    tmp_root.mkdir()
+
+    job = SimJob(detector="pbwo4", config=None, shard_index=5)
+    result = run_job(job, fake, 10000, tmp_path, "steps", "gen1", tmp_root)
+
+    assert result.dest is not None
+    payload = json.loads(result.dest.read_text())
+    assert payload["seed"] == str(job_seed("steps", "gen1", job))
+
+
 def test_run_job_moves_output_to_correct_shard_path(tmp_path):
     fake = _write_fake_executable(tmp_path / "fake_exe.py")
     gen_dir = tmp_path / "raw" / "steps" / "gen1"